The present invention relates to an automatic gain control device for an amplifier capable of setting a gain, an automatic gain control method, a control program for an automatic gain control device, a radio communication terminal having an automatic control device, a radio communication system, and a radio communication method and, more particularly, to an automatic gain control device for an amplifier in the reception unit of a radio communication terminal which can switch the frequency of a reception signal in the compressed mode, an automatic gain control method, a control program for an automatic gain control device, a radio communication terminal having an automatic control device, a radio communication system, and a radio communication method.
In general, in a radio communication terminal such as a PHS, a cellular phone, an information terminal, e.g., a PC capable of wireless LAN connection, or a PDA, AGC (Automatic Gain Control) is performed to improve the precision in converting a reception signal (analog signal) into a digital signal.
In general, in a radio communication terminal, the reception signal received at the antenna must be converted into a digital signal. The reception signal is amplified by an amplifier, and then input to an ADC (Analog Digital Converter) which converts an analog signal into a digital signal. A processor or the like then, for example, extracts communication data and control data from the reception signal converted into a digital signal by the ADC and processes them. Information processing for the reception signal is performed by the processor or the like.
Since the resolution of the ADC is fixed, in order to convert an analog signal into a digital signal with high precision by using the limited resolution as much as possible, an input value to the ADC is preferably a value close to the maximum value in the range of input values (dynamic range). If, however, an input value to the ADC exceeds the dynamic range, proper measurement cannot be done. It is therefore also necessary for an input value to the ADC (an output from the amplifier on the input stage of the ADC) not to exceed the maximum value. On the other hand, the strength of a reception signal varies with changes in external environment.
In order to make the input amplitude of the ADC constant by absorbing changes in the strength of reception signals, the gain of the amplifier must be controlled. In this case, control to make the output of the amplifier capable of setting a gain value constant is called automatic gain control, and an automatic gain control device is a control device including an automatic gain control function.
The performance of an automatic gain control device has a great influence on the signal reception characteristics of an overall radio communication terminal. A high-performance automatic gain control device is indispensable to a high-performance radio communication terminal. The radio signal receiver of the radio communication terminal is exposed to abrupt changes in reception environment for radio signals depending on the movement of the terminal itself, the movement of an electric wave obstacle, and the specifications unique to a radio system. The automatic gain control device is required to stably hold a constant amplifier output even at the time of such a change in environment.
Various techniques concerning automatic gain control have currently been proposed.
First, there is available a technique concerning an AGC method in data communication using a frequency-hopping scheme (Japanese Patent Laid-Open No. 2000-236286). A data signal comprises a sync signal and information signal. AGC is operated during the reception of a sync signal, and stopped during the reception of an information signal. This makes it possible to prevent AGC from producing operation noise during the reception of an information signal, and hence prevents the transmission quality from being influenced by noise. After frequency hopping, the gain value of AGC which is stored before frequency hopping is used.
Second, there is available an AGC technique in the TDMA scheme (Japanese Patent Laid-Open No. 2002-076996). The TDMA scheme is a communication scheme of allocating time-divided data to users. If the number of users is small, there is a time-divided area (time slot) in which no data is transmitted. When AGC is operated in a time slot during which no data is transmitted, AGC operates in a direction to increase the degree of amplification because there is no input signal. When the radio receiver receives again a time slot in which data destined to it, the degree of amplification of AGC becomes inappropriate. For this reason, the radio receiver activates AGC only in a time slot in which data destined to itself is stored, but stops activating AGC in other times. The receiver uses the amplifier setting corresponding to the immediately preceding time slot. This makes it possible to obtain a stable amplifier output.
Third, there is available an AGC technique in the TDMA scheme (Japanese Patent No. 3064081). The predicted value of the gain value of AGC is calculated on the basis of the electric field strength of a specific signal in the immediately preceding time slot. Starting AGC by using the predicted value can perform control using a value close to an appropriate gain value from the beginning of AGC.
Fourth, there is available an AGC technique at the time of high-speed data communication in the CDMA/TDD scheme (Japanese Patent Laid-Open No. 2003-124759). The amount of variation in reception power (RSSI) of the immediately preceding time slot within the same frame is measured on the basis of the data rate, ISCP (Interference Signal Code Power) difference, and Doppler frequency. If the variation is large, the gain value of AGC is determined with reference to the RSSI of the immediately preceding time slot. If the variation is small, the gain value of AGC is determined with reference to the RSSI of a corresponding time slot in the immediately preceding frame. This makes it possible to perform control such that the gain value of AGC converges promptly even if a variation in RSSI is large.
Fifth, there is available an AGC technique based on positional information using a GPS system (Japanese Patent Laid-Open No. 2003-209477). A reception signal level is predicted from the distance between the current positional information of an AGC circuit which has been measured by using a GPS system at the start of communication and a known communication partner station. More specifically, a reception signal level is predicted on the basis of distance information, and the gain value of AGC which is suited to the reception signal level is predicted. ACG operation is stabilized by using the predicted optimal gain value of the amplifier.
The WCDMA standard technology includes the compressed mode as a mode unique to the system (e.g., “3GPP TS 25.212 V4.6.0, pp. 52-55” [online], Sep. 27, 2002, 3GPP, [retrieved Mar. 24, 2004], Internet <URL:
http://www.3gpp.org/ftp/Specs/html-info/25212.htm>:
“W-CDMA Mobile Communication Systems”, edited by Keiji Tachikawa, Maruzen, Jun. 25, 2001, pp. 140-141). The compressed mode is a technique of preventing a decrease in data transmission amount due to a transmission gap, which is a time zone provided by a radio station at the time of transmission of a radio signal, in which no radio signal is transmitted, by transmitting radio signals obtained by compressing data before and after the transmission gap. In general, the compressed mode is assumed to be used for different frequency monitoring (measuring a radio signal having a frequency different from that of a radio signal received in a normal state) which is required when the radio communication terminal performs handover between stations with different frequencies or performs handover between radio systems with different frequencies.
Owing to demands for reductions in manufacturing cost, size, and the like, a radio receiver mounted in a radio communication terminal corresponds to only one frequency. Therefore, in order to receive radio signals with two different frequencies, the tuning frequency of the radio receiver must be changed.
In this case, consequently, when different frequency monitoring is to be performed by using the compressed mode, the radio communication terminal needs to switch the tuning frequency of the radio receiver. More specifically, the radio communication terminal must change the tuning frequency of the radio receiver from the initial frequency to a monitor target frequency at the occurrence of a transmission gap, and must monitor the electric field level, noise level, signal timing, and the like of the reception signal with the monitor target frequency. When the transmission gap is over, the radio communication terminal must change the tuning frequency of the radio receiver to the initial frequency. On the other hand, according to the WCDMA standard technology specifications, the minimum transmission gap in the compressed mode is very short, only 2 ms.
Although it depends on the specifications set by a management company for a WCDMA system, a radio communication terminal or network generally determines, on the basis of information obtained by a plurality of number of times of different frequency monitoring, whether or not the radio communication terminal performs handover.
The WCDMA standard technology requires a technique of switching the tuning frequency of a radio receiver a plurality of number of times within a short period of time. In addition, since only a short period of time is given for different frequency monitoring, and different frequency monitoring operation must be finished within the period of time, an automatic gain control device is required, which promptly converges to an optimal state and becomes stable immediately after switching of the tuning frequency of the radio receiver.
In the first to fifth techniques, however, no automatic gain control device is disclosed, which is based on the assumption that the tuning frequency of a radio receiver is switched a plurality of number of times in a short period of time. Furthermore, there is not disclosed any technique of performing automatic gain control by predictive computation suitable for the operation of switching the frequency of a reception signal.